Influence of the Gate Bias and Temperature on Positive Charge Generation in TFT Gate-Quality Amorphous Silicon Nitride F
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INFLUENCE OF THE GATE BIAS AND TEMPERATURE ON POSITIVE CHARGE GENERATION IN TF" GATE-QUALITY AMORPHOUS SILICON NITRIDE FILMS
Jerzy Kanicki and Mythili Sankaran IBM Research Division Thomas J. Watson Research Center, P.O. Box 218, Yorktown Heights, New York 10598 ABSTRACT
We report on the illumination time dependence of the generation of positive charge in gate-quality nitrogen-rich amorphous silicon nitride films subjected to sub-bandgap illumination at different temperature in vacuum. The influence of film thickness and gate bias applied during illumination on the generation of positive charge is also described. We have found that a stretched-exponential function, which characterizes dispersive charge transport in silicon nitride, gives the best description of our experimental results. INTRODUCTION
The first room temperature observations of metastable positive charge [1-3] induced by sub-bandgap optical excitation in nitrogen (N)-rich silicon nitride was made by D. Jousse and J. Kanicki in 1989. Later on we have shown that the light-induced generation of positive charge can be best described by a stretched-exponential function, which characterizes dispersive charge transport in silicon nitride films [4]. In addition, we have also demonstrated that the positive charge can be photo-bleached to a certain extent by illuminating the nitride with light of lesser energy than needed for the charging process [5]. Although the reported experimental results have established the existence of positive charges and have provided an equation describing their generation, they did not address the influence of various parameters on their generation kinetics. In this paper we report on the influence of film thickness, temperature, and gate bias applied during illumination on the generation kinetics of positive charge. Results obtained during this study should help in providing a better understanding of the microscopic origin of positive charge in silicon nitride films. EXPERIMENTAL
Our studies were carried out on hydrogenated amorphous N-rich silicon nitride (a-Si N 1 .6:H) films of various thicknesses (d) deposited by plasma-enhanced chemical vapor deposition from an ammonia-silane gas mixture (in a 12:1 ratio) at a substrate temperature of 400'C [6]. The resulting nitride has a total hydrogen content of 31 at.%, an optical bandgap of about 5.5 eV, and a 1 MHz dielectric constant of about 6.9±0.3 [4,6] Our results are based on flatband voltage shift ( AVLFB) in the capacitance (C) - gate voltage ( VG) characteristics for Al-nitride-crystalline silicon (MNS) structures subjected to monochromatic (4.13 eV light) sub-bandgap illumination under vacuum through a semitransparent (50 % transmission at 4.13 eV) Al gate electrode. The AVLFB was determined from the C-V curves in comparison to the dark condition (AVLFB = VTFB - VDFB, where VTFB is the flatband voltage after illumination, and VDFB is the flatband voltage before illumination). The incident photon flux was about 2.28x10 12 photons/cm 2 . The illumination was done at different
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